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Magnetic Tweezers

Magnetic tweezers consist in a set of magnets producing a strong field
gradient which is used to exert a force on magnetic beads. The force
is varied by modulating the position of the magnets relative to the
the bead. Usually, magnets are millimeters in size whereas the bead is
only allowed to move by a few microns thus the magnetic force applied
is constant for a given position of the magnets.

A very interesting feature of the magnetic tweezer is its ability to
rotate the magnetic bead, allowing one to easily twist the molecule
which tethers the bead to the sample

Magnetic tweezers intrisincally apply a constant force to a molecule
whose extension varies in response to this stress. This extension is
usually measured by tracking in video the position of the bead using
videomicroscopy.